There is also a reduction in the GFR (glomerular filtration rate) which results from the decreased numbers of glomeruli (filtration structures in the kidneys), cumulative damage in the remaining glomeruli, and reductions in the renal blood flow.
As a result of a decline in GFR, the elderly excrete drugs more slowly and, therefore, at a greater risk for drug overdose. The decrease in GFR makes it difficult for the elderly to excrete excess blood volume, so any IV fluids must be administered slowly and carefully.
The distal portions of the nephron and collecting system become less responsive to ADH (antidiuretic hormone). This means less water and sodium ions are reabsorbed and more potassium ions lost in the urine.
As the tubules become less responsive to ADH, there is a tendency to lose too much water. This rapidly leads to dehydration.
There is a gradual decrease in the total body content of water. As a result, there is an increase in the concentrations of waste products, toxins, and medications. Keeping well hydrated prevents this from happening. However, too much fluid intake can cause other problems. Overhydration may contribute to heart failure in the elderly.
Sphincter muscles lose their tone and become less effective at voluntarily retaining urine, which leads to incontinence involving a slow leakage of urine.
In addition, the ability to control urination is often lost after a stroke, Alzheimer’s disease, or other CNS disorders affecting the cerebral cortex or hypothalamus.
In males, urinary retention may develop secondary to an enlargement of the prostate gland. In this condition, swelling and distortion of surrounding prostatic tissues compress the urethra, restricting or preventing the flow of urine. An enlarged prostate also increases the urgency for urination.
As the aging urinary bladder shrinks and changes in the muscular contractions take place, the bladder becomes less agile in its ability to contract and relax. As a result, the elderly must void more frequently.
Because of the less effective bladder contraction and residual urine, the incidence of bladder infection increases. Also, the weakening of the external sphincter and a decreased ability to sense a distended bladder increase the incidence of bladder incontinence.
The ability to reabsorb glucose and sodium is also diminished. The presence of excess solute (sodium and glucose) in the urine contributes to excess urination and water loss. In addition, impaired reabsorption of glucose interferes with blood glucose monitoring in diabetes.
The kidney tubules are less efficient in the secretion of ions, including the hydrogen ion. As a result, the elderly experience difficulty in correcting acid/base balances.
With immobility and diminished exercise, calcium moves from the bones into the renal tubules where it precipitates, causing kidney stones.